A Boiling Water Reactor (BWR) generates steam in its core. This core is composed of an array of side-by-side, vertically upstanding square sectioned fuel bundles. These fuel bundles divide the core region of the reactor into the so-called core bypass region exterior of the fuel bundles, and the core region interior of the fuel bundles. The flow region interior of the fuel bundles is at a higher pressure than the bypass region. Typically, water is forced to circulate through the fuel bundles by pumping. The flow region exterior of the fuel bundles contains non-boiling water and is used to provide increased presence of water for the moderation of high speed neutrons to low speed neutrons so that the chain reaction in the boiling water reactor can continue.
Fuel bundle construction can be summarized in a simplified format sufficient for the understanding of this invention. A fuel bundle consists of a group of fuel rods between an upper tie-plate and lower tie-plate. The upper tie-plate and the lower tie-plate and the fuel rods extending therebetween are provided with a polygon section, which section is preferably square. This section is surrounded by a water impervious channel which forms a water tight boundary from the lower tie-plate to the upper tie-plate.
In a BWR, the fuel channels perform three distinct and separate functions. First, the channels form individual coolant cells in which the fuel rods or fuel assemblies are located, thus separating boiling coolant from moderator coolant within the core region. Second, adjacent channel sides form the control rod blade guiding annulus. Third, the channels position and laterally support the fuel assemblies.
BWR fuel assembly size has remained basically the same for approximately three decades. A typical size is 5.518 by 5.518 inches square by 166.9 inches long. Initially, bundle size was determined by the capability of the reactivity control system to keep the core in a state of cold shut-down with sufficient reactivity margin, considering one control rod projecting out of the core. This size was appropriate considering the known D-lattice, C-lattice and N-lattice designs in which one cruciform control blade is inserted between every tour bundles in the core. More recent advances in the use of burnable poisons and axial enrichment variation have made it possible to increase the fuel assembly size beyond the typical BWR size. Furthermore, by designing the core such that two control blades are adjacent to two opposite corners of the fuel assembly during shut-down, the bundle size can be increased even more from the viewpoint of reactivity control. This arrangement is called the K-lattice core. The bundle width for this application is approximately 6.375 inches or slightly bigger than the bundle width for other lattices as noted above.
Recently, there has been interest in further increasing the size of the BWR fuel bundle for future BWR plant designs. The motivation for increasing the size is to reduce the total number of control rod drives (CRD's) required for reactivity control, and to reduce the amount of fuel handling and shuffling during the refueling outage. Bundle widths as much as two times the typical existing BWR bundle pitch are under consideration. The invention here explains how such an enlarged bundle can be constructed so that fuel design issues including channel bulge and availability of two phase flow thermal-hydraulic test data are addressed.